1. Field of the Invention
The present invention relates to an expanding screw for an expandable dowel sleeve and including a shaft provided with rotation-transmitting means at its first end and at least one screw thread extending from its opposite second end.
2. Description of the Prior Art
For attachment of add-on parts, in particular for a through-assembly, anchorable attachment devices, which include an expendable dowel sleeve and an expanding screw that serves as an expanding member for the dowel sleeve, are used in solid and hollow constructional components. A dowel sleeve, which is formed, e.g., of a plastic material, is inserted in a preliminary formed, in a constructional component, borehole and is widened with the expanding screw that is driven or screwed in the dowel sleeve. The dowel sleeve material is displaced toward the borehole wall, producing a friction-/formlocking connection of the dowel sleeve with the borehole wall. In order to achieve high load value with the attachment, it is desirable that expansion forces, which are generated by the expanding screw, were high.
In order to increase expansion forces, German Publication DE 30 12 177 A proposes an expanding screw in which the core diameter of the screw shaft is greater than the core diameter of the shaft in the region of the thread section of the expanding screw, and the axial extent of the expansion region of the dowel sleeve is longer than the axial extent of the thread section. In the set condition, the section of the expanding screw having a larger diameter project into the expansion region of the dowel sleeve, permitting to realize high expansion forces.
The drawback of the expanding screw of DE 30 12 177 A consists in the absence of a tooth engagement of the expanding screw with the dowel sleeve. As a result, the attachment can become loose even upon application of a small load, in particular with hollow constructional components such as, e.g., encountered in hollow brickwork.
German Publication DE 34 29 585 discloses an expanding screw for an expandable dowel sleeve and including a shaft having first and second ends, rotation-transmitting means provided at the first end and at least one screw thread extending from the second end. At a distance from the second end, the expanding screw has a thread section in which the screw thread has a greater outer diameter than in the region between this thread section and the second end. The greater outer diameter of the thread in the region of the thread section, which is spaced from the second end, provides for a tooth-like engagement of the expanding screw with the dowel sleeve, preventing the danger of the screw passing through the dowel sleeve.
The drawback of the expanding sleeve of DE 34 29 585 A1 consists in a very small, hardly noticeable increase of the torque that need be applied for screwing the expanding screw in the dowel sleeve. When the expanding screw has, as shown in DE 34 29 585 A1, a screw head at the first end of the shaft, and the screw is completely screwed in, upon application of a further torque, the screw would rotate at the same place, which would damage the dowel sleeve at least in some area and would reduce the obtainable holding values. Also, the preload of the attachment can disappear, so that the add-on part would remain only loosely secured to a constructional component. Furthermore, with the design of the expanding screw according to DE 34 29 585 A1, manufacturing costs of producing such an expanding screw are noticeably increased.
Accordingly, an object of the present invention is to provide an expanding screw for an expandable dowel sleeve and an attachment device with such an expanding screw, which would insure a reliable retention of the expanding screw in the dowel sleeve upon generation of high expansion forces, and easy manufacturing of the expanding screw.